This invention relates to a novel electroluminescent device.
Cathodoluminescent phosphors are known to be efficient light emitters when excited with electrons. Many of them are extremely insulating, having a resistivity greater than 10.sup.11 ohm-centimeters and are therefore poor light emitters when excited with an electric field. For example, europium-activated yttrium oxide, which is known to be a rather efficient cathodoluminescent material has a resistivity substantially greater than 10.sup.11 ohm-centimeters. As used herein, resistivity is measured on the bulk material with a current density of 2 milliamperes per square centimeter (mA/cm.sup.2 ).
In a search for new red emitting electroluminescent materials, electroluminescent films of this oxide were made by radio frequency sputtering on glass substrates coated with a transparent conductive coating. The electroluminescent films, which were about 1500 Angstroms thick, exhibited strong red cathodoluminescence. The films were then coated with a resistive cermet film consisting of a sputtered film about 2 micrometers thick of silica containing about 10 volume percent of nickel metal. Aluminum metal electrodes were then sputtered upon the cermet film. When a dc voltage was applied to the yttrium oxide film, a dim red electroluminescence was observed. Brightnesses of about 3 foot lamberts were obtained, apparently because the films were electrically insulating and relatively small currents could be passed through the film before its dielectric breakdown. Higher conductivity in the luminescent film is apparently necessary to increase the current through the film and to increase the brightness therefrom.